Effects of -ray irradiation on the high temperature oxidation of stainless steel in supercritical water

Nakahara, Yukio ; Yamamoto, Masahiro ; Kato, Chiaki   ; Kiuchi, Kiyoshi; Karasawa, Hidetoshi*; Katsumura, Yosuke*

Corrosion experiments of stainless steel (SS) in supercritical water (SCW) have done with a flowing SCW loop including a -ray irradiated autoclave. Three types of commercial-grade austenitic SS, Type 304L, Type 316L, and Type 310S, were examined. The condition of SCW was adjusted to the core environment of a SCW reactor system. Test samples of the SS were immersed in deaerated pure SCW (25 MPa, 550 C). Feed water was circulated at a flow rate of 4 L h. Absorbed dose rates near the surface of the test samples in SCW were estimated at 5-15 kGy h. The time dependence of net weight gains followed parabolic rate law except -ray irradiated Type 316L SS. The net weight gains of -ray irradiated Type 304L SS and Type 310S SS were larger than those of the non-irradiated. Rate constants of parabolic rate law in net weight gains of Type 304L SS and Type 310S SS were decreased as the -ray exposure rate was increased. The net weight gain of -ray irradiated Type 316L SS was fluctuating and decreasing widely, because the exfoliation of the surface oxide occurred. On the surface of all the samples, the formation of two-layer oxide film in which the outer layer was porous iron oxide and the inner layer was dense iron-chromium-nickel oxide occurred. Iron oxide in the outer layer was magnetite (FeO) with non-irradiated samples. Hematite (-FeO) was additionally formed in the outer layer with -ray irradiated samples. A chromium-rich part in the inner layer was formed along to the oxide/metal interface of -ray irradiated Type 304L SS.